For example, when an outdoor unit of an air conditioner is attached to a concrete wall, a hole is first drilled in the concrete wall, an anchor is attached to this pilot hole, and the outdoor unit is screwed and fixed to the anchor.
A shank is attached to a tip of a drilling tool for forming such a pilot hole, and a bit is provided at a tip of the shank. Further, the bit is rotated, and its tip is pressed against concrete, thereby drilling a hole while cutting contact portions of the concrete. The bit will be worn out in the course of repetition of drilling operations, and will therefore need replacement. A conventional bit has been integrally formed with a shank, but a shank part does not have to be replaced in a normal situation. Currently, there is provided a structure in which a shank and a bit are separately formed and the bit is connected to the shank via screws, thus allowing only the bit to be attached/detached to/from the shank (JP-A-2007-30045). Therefore, when the bit is worn out, it is only necessary to replace only the bit, and the shank does not have to be replaced. Hence, the cost for replacement can be kept at a low level. It is to be noted that the shank is attachable/detachable to/from a main body shaft at a tip of a tool main body via a screw connection, and is normally detached from the tool main body during a non-use period.
Actually, in the structure in which the shank and bit are connected to each other via screws in this manner, a deviation might occur between an axial center of the shank and that of the bit. If the axial centers of the shank and bit do not coincide with each other, the bit deflects in a manner that creates a conical shape while being rotated with respect to the shank, and therefore, the tip of the bit is moved around its rotation axis in a swiveling manner. Consequently, drilling efficiency is reduced, and the hole diameter of a pilot hole becomes greater than the diameter of the bit. Hence, there occurs a problem that the strength of anchor attachment is degraded. It should be noted that a similar problem also occurs when a screw connection is made between a shank and a main body shaft of a tool main body.
Therefore, the center alignment of a shank and a bit is carried out by screwing of screw portions. By tightening the screw portions, the center alignment is enabled. Furthermore, the perpendicularity of shafts is provided by an abutting surface of one of the shafts. More specifically, an end face of a front-end tubular region of a female screw portion is abutted against a bearing surface of a base of a male screw portion, thereby providing the perpendicularity. The center alignment and perpendicularity are also provided similarly when a screw connection is made between the shank and the main body shaft of the tool main body.
A slight gap exists between the male screw portion and the female screw portion, and this gap is inevitable since it is necessary for screw tightening and loosening. However, when a gap exists between the shank and bit, a deviation might occur between the axial center of the shank and that of the bit. If the axial centers of the shank and bit do not coincide with each other, the bit deflects while being rotated when the axial center of the shank and its rotation axis coincide with each other, and therefore, the tip of the bit is moved around its rotation axis in a swiveling manner. Accordingly, drilling efficiency is reduced, and the hole diameter of a pilot hole becomes greater than the diameter of the bit. Hence, there occurs a problem that the strength of anchor attachment is degraded. It should be noted that a similar problem also occurs when a screw connection is made between the shank and the main body shaft of the tool main body.
However, when the shafts are connected to each other via screws, a slight gap is needed between the male screw portion and female screw portion, and therefore, the center alignment is dependent on screw accuracy. Further, the abutting surfaces of the shafts are located outwardly, and are thus susceptible to disturbances such as flaws. Therefore, there were problems that the processing accuracy of screws has to be ensured and the accuracy of the abutting surfaces needs to be maintained.